This invention relates to gas turbine engines and, more particularly, to a gas turbine engine having a variable fan nozzle for controlling a bypass airflow through a fan bypass passage of the gas turbine engine.
Gas turbine engines are widely known and used for vehicle (e.g., aircraft) propulsion. A typical gas turbine engine includes a core comprising a compression section, a combustion section, and a turbine section that utilize a core airflow to propel the vehicle. The gas turbine engine is typically mounted within a housing, such as a nacelle. A bypass airflow flows through an annular passage between the housing and the core and exits from the engine at an outlet.
Presently, conventional gas turbine engines are designed to operate within a desired performance envelope under certain predetermined flight conditions, such as cruise. Conventional engines tend to approach or exceed the boundaries of the desired performance envelope under flight conditions outside of cruise, such as take-off and landing, which may undesirably lead to less efficient engine operation. For example, the size of the fan and the ratio of the bypass airflow to the core airflow are designed to maintain a desired pressure ratio across the fan during take-off to prevent choking of the bypass flow in the passage. However, during cruise, the bypass flow is reduced in the passage and the fuel burn of the engine is negatively impacted. Since engines operate for extended periods of time at cruise, the take-off design constraint exacerbates the fuel burn impact.
Therefore, there is a need to control the bypass airflow over a wider variety of different flight conditions to enable enhanced control of engine operation and to reduce fuel burn.